The present invention relates to a variable ratio steering mechanism of the rack and pinion type. The characteristics and advantages of rack and pinion steering are well known, as are the advantages of providing a variable steering ratio and it is therefore unnecessary to explain these in the present specification.
Considerable problems have however arisen in devision a practical construction which incorporates variable ratio steering in a mechanism of the rack and pinion type. One approach to a solution to the problem is disclosed in the specification of U.S. Pat. No. 2,865,339, in which a non-round, straight cut pinion engages a "wavy" rack, producing a varying ratio of engagement which recurs at each revolution of the pinion and hence also of the steering wheel. This necessarily requires that the wheels of the vehicle to be steered must be moved throughout their entire range of movement from lock to lock by not more than two turns of the steering wheel, and a very low average ratio results. As a high (numerical) ratio is needed near center to avoid excess sensitivity at speed, the change in ratio with this system is generally in excess of 2:1. This shortcoming can be overcome by the introduction of an intermediate gear between the steering wheel and the pinion to allow more turns of the steering wheel or by modifying the rack as described in U.S. Pat. No. 2,973,658. Such constructions however introduce complexities into the system which add undesirable expense.
In addition to directness of the sterring action, it was found very difficult to achieve a smooth action in a steering mechanism of this kind in view of the sudden transitions in tooth action necessitated by the non-round form of the pinion and high pressure angles. A helical form of the device, which might well have cured this roughness, was obviously impractical.
A somewhat different approach to the solution of the general problem of providing a variable ratio steering mechanism of the rack and pinion type is described in U.S. Pat. No. 3,267,763 and the corresponding British Pat. No. 977,434. In that construction a concentrically mounted driving pinion of circular form is meshed with a rack having teeth of various forms. Teeth of minimum pitch and pressure angle providing a maximum ratio are provided at the center of the rack and teeth of maximum pitch and maximum pressure angle providing a minimum ratio are provided at the ends of the rack, the intervening teeth on each side of the center having a progressive variation of pitch and form. Note that the variation of pitch radius of the pinion in any case is a result solely of the forms of the rack teeth.
A detailed consideration of that form of mechanism has shown that, although it is not limited in the number of steering wheel turns, practical considerations so limit the amount of ratio variation and pattern thereof that little benefit has resulted from its use. In a typical passenger car steering gear employing power assist, optimum steering performance requires a variation of steering ratio of just less than 2:1, a rapid drop of ratio from the center, a "flare" out to a more constant ratio with about 21/2 turns stop to stop. If such characteristics are sought to be obtained by employing the subject mechanism, the following difficulties are posed:
1. A very low pressure angle must be used for the form of the central, most frequently used, teeth to such a degree that considerable weakness in the strength of these teeth results.
2. A very high pressure angle must be used for the teeth at the ends of the rack in order to obtain the desired low ratio in this region, as a result there is a large variation of efficiency, successively, during the engagment of each tooth (as much as 30 percent). The requirement that the driver be able to steer the car in these low ratio regions when the power assist has failed governs how low the ratio may be in this region. Intermittently occurring regions of poor efficiency, such as would occur with this system, obviously impose a limitation in selecting the low ratio.
3. A rapid ratio drop, or change of pressure angle, particularly occurring in the high ratio, low pressure angle region, produces "holes" or cusps in the rack tooth flanks at certain points, and the intensity of surface loading at these points becomes unacceptable. Even if this is avoided by reducing the rate of ratio change to less than that desired, areas of very small radius occur on the rack tooth profiles, which will be points of high wear rate.
The first of the difficulties mentioned above is appreciated in the prior specifications referred to in that it is proposed that in order to increase the strength of the teeth, an intermediate reduction pinion be provided between the steering mechanism and the pinion meshed with the rack. However, as has been pointed out above, this is an expensive solution.
The object of the present invention is to provide a variable ratio steering mechanism of the rack and pinion type, the design of which overcomes these listed difficulties.